What is Hund's rule?

Answer

Hund’s rule states that orbitals are occupied by one electron before it is doubly occupied, and each of the electrons in the singly occupied orbitals have the same spin. Generally, electrons fill the lowest energy levels first, except when it is presented with occupying an orbital by itself. In this case, the electron fills an empty orbital even if it is not the lowest. Hund’s law explains this process.

Electrons group themselves in pairs of two within orbitals. However, because electrons are negatively charged, they repel each other. For this reason, an electron prefers to minimize repulsion by occupying an orbital by itself, which is the first principle of Hund’s rule, which is that electrons always occupy an orbital singly before it pairs with another. The second principle of Hund’s law states that each of the unpaired electrons has the same spin. Paired electrons, however, have opposite spins. For example, one electron spins upward while its partner electron spins downward. Spinning in opposite directions creates balance within orbitals with paired electrons. However, since electrons occupying orbitals singly do not affect the spin of other electrons, they are free to spin in the same direction. Usually, scientists draw unpaired electrons in the “spin-up” direction.

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